This snippet shows you how you can include a message after the Help Settings form:

add_action( 'gfpdf_post_help_settings_page', function() {
/* We set the action priority to 3 so this content will be included before the Uninstaller */
?>
<h3><?php _e( 'Custom Section', 'prefix-text-domain' ); ?></h3>
<p><?php _e( 'This is the message for this new section. You should include any additional functionality here...', 'prefix-text-domain' ); ?></p>
<?php
}, 3 );